Recently, researches have been conducted to develop a memory device having large capacity and capable of overcoming the limit of refresh required in, e.g., DRAM (Dynamic Random Access Memory) device by using a ferroelectric thin film as a dielectric film of a capacitor.
A ferroelectric random access memory (FeRAM) using such a ferroelectric thin film is a kind of nonvolatile memory device and has advantage that it is capable of maintaining stored information even when power is cut. Besides, FeRAM has further advantages such as high speed accessibility, low power consumption, high strength against impact. Due to these advantages, FeRAM is expected to find applications as a main memory in various electronic devices and equipments having file storage/retrieval function, such as a portable computer, a cellular phone and a game machine, or as a storage medium for storing thereon voice or image.
In a FeRAM device, a memory cell composed of a ferroelectric capacitor and an access transistor stores therein data ‘1’ or ‘0’ having a logical state depending on an electric polarization state of the ferroelectric capacitor. If a voltage is applied to both ends of the ferroelectric capacitor, a ferroelectric material is polarized according to a direction of an electric field. A switching threshold voltage at which the polarization state of the ferroelectric material is changed is referred to as a coercive voltage. To read the data stored in the memory cell, voltages are applied to between two opposite electrodes of the ferroelectric capacitor, and the state of the data stored in the memory cell is detected from a variation in quantity of electric charges excited in a bit line.
As candidates of universal memory, there are known a MRAM (Magnetoresistance Random Access Memory), a PRAM (Phase-change Random Access Memory), a RRAM (Resistive Random Access Memory), etc besides the FeRAM.
Through active researches upon a single universal memory combining speed of SRAM (Static Random Access Memory), non-volatility of flash memory and high capacity of DRAM, there are resulted in various recent memory technologies including FeRAM [M. E. Lines and A. M. Glass, Principles and Applications of Ferroelectrics and Related Materials (Clarendon, Oxford, 1977); J. F. Scott, Ferroelectric Memories (Springer, New York, 2000)], MRAM(Magnetoresistance Random Access Memory), PRAM(Phase-change Random Access Memory), RRAM(Resistive Random Access Memory), etc. FeRAM is basically based on electrically switchable voluntary polarization, and gives high performance as well as high speed [D. S. Rana et al., Adv. Mater. 21, 2881 (2009)] and low power consumption [J. F. Scott, Science 315, 954 (2007)].
In FeRAM technology, however, achieving high storage capacity is difficult and takes high cost. For the reason, FeRAMs having relatively low capacity are put on market only for specific application. To overcome this problem, many researchers have attempted to increase a storage capacity per a unit area by, for example, simplifying structure of a device, reducing the size of the device and diversifying two types of polarization states, which are represented by on and off or 0 and 1, into multiple numbers of intermediate polarization states, thus allowing multilevel bits. Among these various researches, it has been attempted to vary a voltage of an electric pulse applied to the device. Korean Patent No. 10-0543198 describes a ferroelectric memory device having a multi reference voltage generator and attempts to diversity polarization states into various reference voltage levels generated in a negative bit line by on/off optional processing of a first fuse and a second fuse. Further, U.S. Pat. No. 7,196,924 B2 describes a method of adjusting polarization in several stages by applying different voltage to a ferroelectric layer, thus allowing FeRAM to have multiple data values.
However, the aforementioned conventional methods for obtaining multilevel polarization states have attempted to realize multi-levels by varying voltage levels which is applied to ferroelectrics. These conventional methods, however, are highly dependent on characteristics of a ferroelectric material and lack reproducibility and reliability. For example, even if multi-levels having four different polarization states are achieved by adjusting an applied electric strength (amplitude of pulse or width of pulse), the degrees of such polarization states may fluctuate whenever the memory is operated. Thus, actually, the four polarization states may not be guaranteed reliably. In such a case, in an actual device composed of a multiple number of individual ferroelectric memory cells, even if each cell has multilevel polarization states, all the cells may not have constant polarization states. Besides, since reliability of generated polarization states may not be guaranteed each time a single storage cycle taking several tens of nanoseconds is repeated, the device may not be qualified to be used.